Of power storage devices, capacitors are widely used for various kinds of electric apparatuses and the like. Among capacitors of many types, an electric double layer capacitor and a lithium ion capacitor have large capacities, and are particularly attracting attention in recent years.
An electric double layer capacitor is a power storage device including a cell, a sealed container for securing electric insulation between cells and preventing liquid leakage, a power collecting electrode for taking out electricity, and a lead wire. Said cell mainly includes a pair of activated carbon electrodes facing each other, a separator for electrically separating them, and an organic electrolytic solution for exhibiting capacity.
Further, a lithium ion capacitor is a power storage device in which an electrode which can electrostatically adsorb and desorb ions, such as an activated carbon electrode, is used as a positive electrode, and an electrode which can occlude lithium ions, such as hard carbon, is used as an negative electrode.
Energy stored in an electric double layer capacitor is expressed by the following equation (1):W=(½)CU2  (1),
where W indicates stored energy (capacity), C indicates electrostatic capacitance (dependent on the surface area of an electrode), and U indicates cell voltage.
From the above equation (1), it is conceivable that improvement in electrostatic capacitance contributes to improvement in stored energy.
Japanese Patent Laying-Open No. 2005-079505 (PTD 1) discloses an “electrode material for an electric double layer capacitor, characterized by being made of a gel composition including: carbon nanotubes obtained by applying a shear force to the carbon nanotubes and subdividing the carbon nanotubes in the presence of an ionic liquid; and the ionic liquid”, to improve electrostatic capacitance in the electric double layer capacitor.
Japanese Patent Laying-Open No. 2009-267340 (PTD 2) discloses an “electrode for an electric double layer capacitor, characterized in that a sheet prepared by molding carbon nanotubes with a specific surface area of 600 to 2600 m2/g in the shape of paper is integrated with a base material which constitutes a power collector and has an irregular portion in its surface, through the irregular portion”.